The field of the invention is programmable controllers such as those described in U.S. Pat. Nos 3,810,118; 3,942,158; and 4,165,534.
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored program. In programmable controllers such as those disclosed in the above cited patents, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence to examine the condition of selected sensing devices on the controlled equipment and instructions which energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
The processor in a programmable controller is designed to rapidly execute programmable controller type instructions, which for the most part call for the manipulation of single-bit input data and the control of single-bit output data. The length of the control program, and hence the complexity of the system to be controlled, must be limited to insure that the entire control program can be executed, or scanned, within a set time. Such time limits are required to insure that the programmable controller will provide virtually instantaneous response to any change in the status of sensing devices on the controlled system. Therefore, the speed with which a controller processor can execute programmable controller instructions has a direct bearing on the size of the machine or process which it can effectively control.
In addition to the more conventional programmable controller instructions, there is an increasing need for an expanded instruction set which will enable programmable controllers to perform functions hitherto associated with general purpose computers. This is particularly true of large programmable controllers which are capable of controlling more than one industrial process, gathering management data, and generating management reports. It is important, however, that such functions not degrade the response time of the controller.
Another requirement of programmable controllers is that they be flexible in terms of the number and types of I/O devices with which they will operate. There are numerous types of input devices available ranging from simple switches to a variety of analog to digital converter devices. Similarly, output devices range from simple lights and solenoids to a wide variety of digital to analog conversion devices. There is a wide range of protocols, and data types associated with such I/O devices and the user should be provided with the means for using these devices without special hardware interface circuits and complex data conversion requirements.